To navigate through our surroundings, we must accurately perceive our direction of self-motion (i.e., heading). Heading perception is an interesting problem in sensory integration, requiring neural circuits that combine visual motion signals (optic flow) with vestibular signals, and perhaps also somatosensory and proprioceptive cues. The multi-sensory nature of heading perception can be appreciated by the experience of section, the powerful illusion of self-motion that accompanies large-field visual motion (e.g., at an IMAX theater). Although the processing of optic flow has been well studied in visual and parietal cortices, little is known about how or where visual and vestibular signals are integrated for heading perception. Areas MSTd and VIP appear to be promising candidates, for these areas are known to be involved in processing optic flow and have also been found to contain vestibular signals regarding head translation. The proposed experiment, which employ a custom-designed virtual reality system, address three specific aims regarding the neural basis of heading perception in trained primates. Aim #1 examines the relative contributions of visual and vestibular cues to heading selectivity in MSTd/VIP. Specifically, we test whether the heading activity of neurons is enhanced by congruent combinations of visual and vestibular cues. Aim #2 tests whether heading signals derived from visual and vestibular cues are coded in a common reference frame (eye-centered, head-centered, or intermediate), as might be expected if these different sensory signals, are combined synergistically to improve heading selectivity. In Aim #3, we test more directly whether MSTd and VIP contribute to heading perception by recording from neurons during performance of a heading discrimination task. Monkeys will perform this task using optic flow alone, vestibular signals alone, or congruent combinations of the two cues. This will allow us to test whether MSTd/VIP neurons can account for the improvement in heading sensitivity seen under cue combination. These experiments will provide a comprehensive examination of whether MSTd/VIP neurons are involved in sensory integration for heading perception. Of clinical relevance, heading perception can be severely impaired in Alzheimer's disease, and this may contribute to spatial disorientation and navigational difficulties. By helping to elucidate the brain areas involved in heading perception, this work may eventually aid in targeting new Alzheimer's therapies to the appropriate brain regions.